Journal of Fusion Energy, Vol. 25, No. 3/4, December 2006 (Ó 2006) DOI: 10.1007/s10894-006-9019-4

Utilization of Refractory Metals and Alloys in Fusion Reactor Structures Mustafa U¨beyli1,* and S¸ enay Yalc¸ın2

In design of fusion reactors, structural material selection is very crucial to improve reactor’s performance. Different types of materials have been proposed for use in fusion reactor structures. Among these materials, refractory metals and alloys having capability to withstand high temperatures and high neutron wall loads have been considered to get high power density in fusion reactors. However, these materials have insufficient technological database and are very expensive compared to steels. In addition to that, except chromium and some chromium alloys they show no low activation property. This study gives an overview of potential of refractory metals and alloys for possible use in fusion reactors. KEY WORDS: Fusion; refractory metals and alloys; high power density.

structural material plays a key role in design of fusion reactors.

INTRODUCTION Controlled fusion energy has potential in providing unlimited energy for mankind. A fusion energy system has attributes of an attractive product with respect to safety and environmental advantages compared to other energy sources [1,2]. Furthermore, fusion fuels are abundantly available in the nature, contrary to relatively scarce fission fuel resources. Therefore, there have been many studies focused on fusion energy research in the past 40 years. However, there are many obstacles that must be overcome to reach a commercial fusion reactor. One of them is the development of suitable structural material or design concept to eliminate or reduce frequent replacement of first wall structure during reactor’s lifetime and to reach high power density in the reactor to become competitive. For this reason, selection of suitable

Competitiveness of Fusion Reactors A commercially competitive fusion reactor requires high power density (HPD), high power conversion efficiency (>40%), high availability (lower failure rate, faster maintenance) and simpler technological and material constraints. These represent primary goals for fusion power technology (FPT) [3,4]. The two most important requirements for obtaining practical HPD systems are:  High power production per unit volume of the plasma;  FPT in-vessel components that can handle the high surface heat flux and high neutron wall load (NWL) on the first wall in such HPD systems [3]. In this case, the neutron flux load on the first wall becomes a key issue.

1

Engineering Faculty, Mechanical Engineering, TOBB Economics and Technology University, So¨g˘u¨to¨zu¨ Cad. No: 43, 06530, C¸ankaya-Ankara, Turkey. 2 Engineering Faculty, Computer Engineering, Bahc¸es¸ehir _ University, Istanbul, Turkey. * To whom correspondence should be addressed. E-mail: [email protected].

For a breakthrough into the energy markets, fusion reactors must compete primarily with fission 197 0164-0313/06/1200-0197/0 Ó 2006 Springer Science+Business Media, Inc.

U¨beyli and Yalc¸ın

198 reactors. Abdou and the APEX team have investigated the key elements for a competitive fusion power system [3,4]. It can be seen from Table 1 that the average core power density in a fission reactor is much higher than in an International Thermonuclear Experimental Reactor (ITER) type reactor by a factor of 80, 7.5, and 200 for Pressurized Water Reactor (PWR), Boiling Water Reactor (BWR), High Temperature Gas-Cooled Reactor (HTGR), and Liquid-Metal Fast Breeder Reactor (LMFBR), respectively. If fusion reactors are to achieve the same average power density, the NWL will need to be in the range 22–600 MW/m2. Nevertheless, such high wall loads may be impossible to achieve and handle in current magnetic fusion concepts. On the other hand, fusion can be expected to be acceptable at a somewhat higher cost than fission due to its environmental and safety advantages. Fusion research should set a preliminary goal for the NWL to be greater than 10 MW/m2 in order to enhance the potential of economic competitiveness for fusion power systems [3]. In order to reach such high NWLs in fusion reactors, either flowing liquid wall between plasma and first wall or refractory metals and alloys as a first wall structure should be used. Material Selection for Fusion Reactors Fusion reactor first walls around the fusion chamber must withstand high energetic charged particle fluxes, Bremsstrahlung and gamma-ray radiation, and high energetic intense neutron fluxes with a mean energy 14 MeV, which are expected to lead to much higher material damage than observed by fission reactors. In addition, fusion reactors have been expected to operate at higher temperatures and use chemically aggressive coolants such as natural lithium, Li17Pb83, Li25Sn75, and Li2BeF4. Therefore, structural materials of fusion reactors, especially first walls, must withstand thermal, mechanical, chemical,

and radiation loads under fusion neutron environment. General requirements for the materials considered to be used in a competitive fusion reactor can be given as below [4,5]:  Attractive high temperature physical and mechanical properties.  Low activation from 14 MeV neutrons.  Low neutron absorption cross sections.  High NWL capability.  Resistant to helium and hydrogen produced by nuclear reactions.  High thermal conductivity independent of radiation damage level.  Structures must be suitable for shallow burial after decommissioning of the power plant.  Broad compatibility with cooling fluids and gases.  Easy fabrication with different processes.  Resistant to 14 MeV neutrons induced displacement damage.  Low swelling or void formation.  Low cost.  Availability. Unfortunately, there is no unique material satisfying all these needs listed above. However, different types of materials have been proposed to be used in fusion reactors. They can be classified into three main groups given below: 1. Conventional austenitic steels [6–13]. 2. Low activation materials (ferritic/martensitic steel [14–22], vanadium alloys [23–33], SiC/SiC composites [34–39], modified austenitic steels [40–44]). 3. Refractory metals and alloys [45–79]. Refractory metals and alloys are seemed to be important structural materials for fusion reactors due

Table 1. Power density and heat flux in fission reactors compared to a fusion reactor design [3]

Core length (m) Average core power density (MW/m3) Peak-to-average heat flux at coolant interface a

PWR

BWR

HTGR

LMFBR

3.8 96 2.8

3.8 56 2.6

6.3 9 12.8

0.9 240 1.43

Fusiona at 3 MW/m2 15 1.2b 50c

Based on a tokamak power reactor of the ‘‘ITER-type,’’ where the fusion power is scaled up from ITER by a factor of 3, corresponding to a neutron wall load of 3 MW/m2, while keeping the reactor volume the same. b Average core power density is obtained using only the volume of materials in the in-vessel components and the magnets. The volume of ‘‘void’’ regions such as the plasma is excluded. c Peak is at the divertor.

Utilization of Refractory Metals Alloys in Fusion Reactor Structures to their capability of handling higher NWLs and operation temperatures in comparison to other candidates. This study presents the overview of these materials considered to be used in fusion reactors.

REFRACTORY METALS AND ALLOYS The group VB consists of V, Nb, and Ta while Cr, Mo, and W belong to the group VIB of periodical table. All of the group VB refractory alloys are ductile at room temperature, whereas the group VIB refractory alloys are generally brittle at room temperature. Therefore, group VB metals are relatively easy to fabricate into various shapes, whereas the group VIB metals are very difficult to fabricate. Either group VB metals or group VIB metals have BCC structure so that these materials show a transition from ductile to brittle fracture behavior depending on temperature. In addition to that, all of them have oxidation problems at elevated temperatures ( >500°C) [80]. Vanadium and its alloys due to their low activation and some distinct properties have been considered outside of refractory alloy group. The most important advantages of refractory alloys over the low activation materials, namely, ferritic steels, vanadium alloys, and SiCf/SiC are their much higher operating temperatures and higher NWL capabilities (Figures 1 and 2). Therefore, niobium, tantalum, chromium, molybdenum, and tungsten alloys with pure chromium and pure tungsten have been proposed as potential candidates for high performance in fusion reactors. However, they do not satisfy the ‘‘low activation’’ criteria except chromium and some chromium alloys.

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Niobium Alloys Nb–1Zr alloy is the primary niobium alloy considered as structural material for fusion reactors [45,46]. On the other hand, some other niobium alloys, FS-85 (Nb-28Ta-10W-1Zr) and B-88 (Nb28W-2Hf-0.067C), may have potential to be used like Nb–1Zr due to the fact that they have much higher strength than Nb–1Zr and FS 85 has a very good weldability, fabricability, and creep resistance [46]. It has a temperature window for efficient utilization in nuclear reactors with a maximum operating temperature of 1100°C by considering thermal creep and a minimum operating temperature of 500°C by taking into account of irradiation hardening effect for Nb– 1Zr. In addition, its thermal conductivity, increasing with increased temperature, is 52 and 65 W/m K at room temperature (RT) and 600°C, respectively. Moreover, the maximum neutron wall load which is limited by the stress criterion for Nb–1Zr is proposed as 6.6 MW/m2 [3]. At RT, Nb–1Zr has an elastic modulus of 104 GPa and a yield strength of 200 MPa. The strength values for this alloy changes with temperature. A modest decrease in the yield strength of Nb–1Zr up to 400°C is observed, and then it becomes almost constant between 400 and 1100°C (ry = 90 MPa). Finally, it decreases gradually from 90 MPa at 1100°C to 50 MPa at 1400°C. Nb–1Zr shows good ductility at about 600°C and it has reasonable creep strength at a temperature range of 600–700°C [79]. According to limited available data, Nb–1Zr shows very good corrosion resistance in lithium bearing coolants. It is very stable in Li17Pb83 eutectic alloy and dissolution rates are very low even at 600°C

1600 14 12

1200

NWL (MW/m2)

Temperature (°C)

1400

1000 800 600 400

10 8 6 4

200 2

0 1

2

3

4

5

6

7

8

Fig. 1. Maximum operation temperature of the candidate structural materials considered for fusion reactors; (1) Ferritic steel, (2) Oxide Dispersed Steel, (3) V–Cr–Ti, (4) SiC/SiC composite, (5) Nb–1Zr, (6) TZM, (7) T-111, (8) Tungsten.

0 1

2

3

4

5

6

7

8

Fig. 2. Maximum NWL limit for the candidate materials; (1) Ferritic steel, (2) SiC/SiC composite, (3) Oxide Dispersed Steel, (4) V–Cr–Ti, (5) Nb–1Zr, (6) Tungsten, (7) T-111, (8) TZM.

U¨beyli and Yalc¸ın

200 [48]. Moreover, this alloy is compatible with liquid lithium up to 1000°C under static or dynamic loops. Furthermore, it is suggested that Flibe may have good compatibility with Nb alloy [49]. However, Nb alloys have embrittlement tendency in presence of hydrogen [50]. There is a lack of data about irradiation properties of Nb–1Zr alloy. Void fraction in Nb–1Zr alloy irradiated by neutron fluences from 1 to 8  1026 n/ m2 (E > 0.1 MeV) changes with irradiation temperature and reaches the maximum value of 2.2% at 800°C. Irradiation by a neutron fluence of 2.6  1026 n/m2 at 450°C, increases the strength of the Nb–1Zr by a factor of nearly 6. And also a drastic decrease in uniform elongation is observed [46]. The utilization of niobium alloys in fusion reactors can be considered only under some restricted conditions due to their high-induced radioactivity causing significant safety and waste disposal problems. Tantalum Alloys Tantalum is another refractory metal that has a very high melting point of 3000°C and a density of 16.7 g/cm3 at room temperature. The most famous tantalum alloy, Ta-8% W-2% Hf (T-111), was developed in the early 1960s and commercially produced for space applications. A good summary for the thermophysical and mechanical properties of T-111 has been made by Zinkle [51]. This alloy has the second rank in peak neutron wall load which is evaluated as 11.6 MW/m2 [3] so that it has become an attractive material for fusion reactors to get high power and efficiency. The thermal conductivity of Ta-8W-2Hf is 42 W/m K at RT that varies with temperature and reaches 56 W/m K at 1350°C [69,71]. The strength of T-111 depends on either the alloy is recrystallized or stress-relieved condition. Stress-relieved T-111 is preferred due to having higher strength in comparison to recrystallized one. The yield strength at RT for the stress-relieved T-111 is 900 MPa while that for the recrystallized one is 600 MPa [69,71,76]. It gradually decreases with increased temperature and becomes 600 MPa and 275 MPa at 1000°C for the stress-relieved and recrystallized T-111, respectively [69,71,76]. The elastic modulus for T-111 is 180 GPa at RT and decreases down to 155 GPa at 1000°C [71]. It is known that T-111 is compatible with liquid lithium at least up to temperatures of 1370°C [72–74]. And also, Ta alloys have also exhibited good

compatibility with other liquid metals, namely Na, K, and Pb at temperatures up to 1200°C [72–74]. The maximum operating temperature for T-111 is proposed as 1200°C since thermal creep becomes serious at temperatures > 1200°C. On the other hand, the minimum operating temperature will be determined by irradiation hardening causing increase in DBTT and embrittlement. The suggested minimum operating temperature for T-111 is around 650°C [51]. Neutron irradiation causes a pronounced increase in the yield and ultimate tensile strength of Ta-8W-2Hf at temperatures below 650°C. An increase of the yield and ultimate tensile strength to 1250 MPa is observed for a specimen tested at 400°C following fast reactor irradiation at 410°C under a neutron fluence of 1.9  1026 n/m2 (E > 0.1 MeV), causing 2.5 displacement per atom (dpa) [75,77]. The unirradiated elongations are 16–22% for test temperatures between 20 and 650°C whereas the uniform elongation decreases to 0.1 MeV). Furthermore, the DBTT of these alloys may increase up to 600–700°C [59]. Fabritsiev and Pokrovsky [57] have investigated some important properties of irradiated and unirradiated Mo–Re alloys with Re varying from 0.5 to 47%. Irradiation to 5 dpa in the temperature range of 450–800°C results a catastrophic embrittlement in both pure Mo and Mo–Re alloys. At Tirr = 750°C, total elongation of the unirradiated pure Mo is found 18% while for the irradiated one is obtained as only

1%. On the other hand, these values are 17% and 3% for the unirradiated and irradiated Mo-5%Re alloy, respectively. All alloys with 0.5% Re—20% Re show nearly a two-fold reduction in the strength properties at Ttest = Tirr = 450–550°C and a zero plasticity. At Tirr = 760–800°C, irradiation of Mo–Re alloys to 5– 10 dpa demonstrate hardening and a modest total elongation of 2–3% [57]. Scibetta et al. [58] have studied the tensile and fracture toughness of both irradiated and unirradiated TZM and Mo-5% Re alloys. According to their experimental results, the yield strength of unirradiated Mo-5%Re is 600 MPa at RT and decreases slowly down to 450 MPa at 500°C. On the other hand, the yield strength of unirradiated TZM material is recorded as 400 MPa at RT and decreases exponentially down to 100 MPa up to a temperature of 200°C. And then it becomes almost steady at a temperature range of 200–500°C. Total elongation for TZM is 40% at RT and increases gradually with increased temperature. Then it becomes 56% at 450°C. Mo-5%Re has a total elongation of 44% and 20% at RT and 450°C, respectively. The irradiation using a neutron fluence of 3.50  1020 and 2.88  1020 n/cm2 (E > 1 MeV) corresponding to 0.35 or 0.29 dpa, respectively on the tensile properties of both materials results a strengthening of the material and a reduction of ductility. Mo-5%Re retains a higher ductility after irradiation, in particular at low temperatures [58]. Tungsten and Tungsten Alloys Tungsten is chosen as the structural material for EVOLVE (evaporation of lithium and vapor extraction) [61] and ITER fusion reactor design concepts [62]. Tungsten and its alloys have superior mechanical and thermal characteristics compared to other candidates. These materials can operate under very high temperature and have low erosion rate at the edge plasma temperature < 40–50 eV. Moreover, they show low tritium permeability. However, W has a weakness of low ductility at the operating temperatures and its manufacture is rather difficult [50]. Among the candidate structural materials, tungsten has the highest operating temperature limit (1500°C). The lowest operating temperature limit is 500°C for tungsten, due to the fact that tensile elongation becomes nearly 0 at < 500°C. In design studies, tungsten allows a maximum neutron wall load of 8.8 MW/m2, which is limited by stress

Utilization of Refractory Metals Alloys in Fusion Reactor Structures criterion [3]. It has a very high thermal conductivity and very low thermal expansion coefficient (see Table 2) [63]. W has an elastic modulus of 410 GPa and a tensile strength of 1000 MPa at RT [63]. Rhenium can be added to tungsten to improve ductility even at lower temperatures, the creep strength and recrystallization resistance. Tungsten has a high solid solubility for Re at elevated temperatures more than 30% of rhenium can be inserted to the tungsten lattice. W-5% Re, a single phase material, still has a fairly high thermal conductivity, compared to rhenium-free tungsten alloys, an excellent thermal shock resistance, and a high strength and good braze capability, as well as some weldability. The manufacture and machinability of W-5Re even at room temperature is satisfactory for the size and shape of high heat flux armor in plasmainteractive components. In the case of recrystallized tungsten–rhenium alloys, the DBTT is RT for 26 wt% Re (W–26Re), linearly increasing up to 350°C when reducing the rhenium content down to 0% (pure tungsten) [63]. The addition of La2O3 to tungsten improves the grain boundary strength at ambient and elevated temperatures, resulting in a remarkable improvement of the thermal shock and creep resistance, in the machinability and hot tensile strength. Although La2O3 additions to tungsten do not reduce the DBTT initially, the material becomes machinable at RT, thus at lower cost and the recrystallization temperature rises by 100–350°C. Recently a weakly alloyed tungsten (W–Mo–Y–Ti) has been developed. The addition of the reactive elements, Y and Ti reduces the amount of free oxygen and carbon, resulting in improved mechanical properties [63]. This alloy shows some tensile elongation even at 100°C [64]. In order to improve both the low temperature toughness and the resistance to embrittlement, ultra-fine grained tungsten alloys with TiC additions of 0.2–0.3% have been developed. The tungsten alloy with 0.2% TiC shows a significant ductility and high strength at and above 180°C and the DBTT of this alloy is lower by more than 100°C than that of pure tungsten [66]. W–4Re– 0.32HfC alloy is developed to improve the high temperature tensile and creep strength of tungsten without affecting the room temperature ductility [67,68]. The creep strength of this alloy is two orders of magnitude higher than that of pure tungsten and one order of magnitude higher than W–5Re at temperatures between 1900 and 2100°C [67]. Tungsten and its alloys show good compatibility with lithium up to 1370°C. And also, tungsten is

203

compatible with Li17Pb83 at temperatures greater than 600°C. There is no certain data about the compatibility of W and W alloy with Sn–Li alloy and Flibe [27]. There is a limited data on irradiation damage of W–Re alloys for structural application. The swelling of tungsten at higher dose rates is strongly reduced for all irradiation temperatures when alloyed with rhenium. The DBTT of W–Re increases when irradiated at lower temperatures [63]. The DBTT of W–10Re increases by about 200°C only by the irradiation of 0.4 dpa at 250–300°C [64]. The superior ductility of W–26Re is destroyed by the rather light damage exposure, comparable to the ITER conditions [66]. Tungsten has also been considered to coat some fusion reactor materials due to its high energy threshold for physical sputtering and its lack of sensitivity to chemical sputtering [63,78]. Good adhesion is obtained when tungsten is coated on graphite, stainless steel and copper [78].

CONCLUSIONS AND DISCUSSION Refractory metals and alloys have advantages of withstanding high NWLs and high operation temperatures. Among these materials, tungsten have also lower cross sections of atomic displacement and helium generations under fusion neutron environment than steel [81], so that its lifetime would be much longer in fusion reactors compared to steels [82–84]. Generally, they seem to be suitable materials to supply high performance in fusion reactors. Nevertheless, technological database for these materials especially on irradiation effect on their mechanical behavior and compatibility properties in Li, Li17Pb83, Sn–Li and Li2BeF4 environment is very scarce. In addition to that, they do not exhibit low activation property except for chromium and some chromium alloys so that they (especially, Mo, Nb, and Ta) have safety and waste disposal problems. Moreover, they are much more expensive than steels. Development of alloys, intermetallics, and composite materials that will be based on the refractory metals and alloys should be carried out to reduce irradiation embrittlement, decrease DBTT to lower temperatures and increase toughness to get improved refractory materials to be used in fusion reactors. Finally, an intense fusion neutron source at 14 MeV must be developed to get more realistic data about the irradiation behavior and lifetime of the refractory metals and alloys as well as other candidate materials.

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